Fix incorrect solutions for problems in 5.1 and 5.2

[adj-matrix]

Here are the main changes:

• Problem 5.1 (Probabilistic Counting): The variance calculation was incorrect. It missed the a^2 factor from the Var(aX) property. The correct variance is 99n, not 0.99n. I've added the correct step-by-step derivation.
• Problem 5.2 (RANDOM-SEARCH): The provided pseudocode implemented sampling without replacement, which contradicts the problem description ("we may examine a given element more than once"). I've corrected the pseudocode to match the problem statement.